The present invention relates to copolymers containing para-alpha-dimethylstyrene, to a thermoplastic composition and its preparation.
The preparation of copolymers of para-alpha-dimethylstyrene (DMS) with further monomers, e.g. acrylonitrile (AN), maleic acid derivatives (in particular anhydride and imines) leads to thermoplastic molding materials with improved properties and in particular to improved heat distortion temperatures. The use of natural sources, like limonene, alpha-, beta-pinenes or other terpenes, to prepare the monomer DMS which has the following formula
is also part of the invention. Also the further monomers, like acrylonitrile, can be prepared starting from natural sources, e.g. from amino acids.
The synthesis of polymers and copolymers from or containing styrene has long been known. A historical overview is outlined e.g. in “Modern Styrenic Polymers: Polystyrene and Styrenic Copolymers” (edited by J. Scheirs and D. B. Priddly, John Wiley and Sons, 2003).
There are several problems involved in pure polystyrene, like e.g. its brittleness and chemical resistance. In order to prepare a thermoformable composition based on the monomer styrene with a higher heat distortion temperature, copolymers of styrene with other monomers were prepared. One commercial important derivative is a copolymer of styrene with acrylonitrile, usual referred to SAN copolymer. The Vicat softening point of such polymers is in the range of 103-120° C., with an average of about 110° C. These polymers are described e.g. in EP-A 022 200, U.S. Pat. No. 4,294,946 and EP-A 0 712 895.
Several strategies are known to improve the Vicat temperature, each however with particular disadvantages. Thus instead of using styrene as a monomer, alpha-methylstyrene was proposed in the literature. It was found years ago that the Vicat temperature of copolymers improved by using alpha-methylstyrene instead of or on addition to styrene. However, the monomer alpha-methylstyrene is not readily polymerized, and remaining monomers are hard to remove from the thermoplastic composition. Furthermore, the copolymers often have an unpleasant smell. The improvement of the property heat-distortion-temperature (HDT) in these copolymers and in economically important blends of SAN and PVC is not so effective as with blends of PVC with copolymers of maleic N-phenyl imide on a weight to weight basis.
The preparation of copolymers from styrene and maleic anhydride or maleic N-imides has been reported to arrive at copolymer materials with high glass temperatures. The copolymer from styrene and maleic anhydride however is very brittle and the copolymers from styrene and maleic-N-imides contain nitrogen and tend to strongly yellow on thermal processing procedures particularly at higher temperatures, which is disadvantageous (see e.g. U.S. Pat. No. 3,642,949 and EP-A 0 721 476).
The use of various mixtures of para-and meta-alpha-dimethylstyrene has been described in the literature. In DE-A 2036420 the polymerization of acrylonitrile with polybutadien and dimethylated styrene is described. Here also an improvement in the glass temperature can be reached. The heat distortion temperature (HDT) of these copolymers was found to be up to 131° C. These copolymers however do not reach the goal to provide with a cheap modifier for polymers like PVC.
A further incentive to reach a suitable copolymer of styrene derivatives and acrylonitrile with a good heat distortion temperature is the fact that styrene today is not considered as being a sustainable monomer. As the sources of oil and gas are more and more limited, monomers are preferred which can be obtained from natural and in particular plant sources.